Field of the Invention
The present invention relates to a wafer processing method for dividing a wafer along a plurality of division lines formed on the front side of the wafer, the wafer being composed of a substrate and a functional layer formed on the front side of the substrate, the functional layer being partitioned by the division lines to define a plurality of separate regions where a plurality of image sensors as devices are individually formed.
Description of the Related Art
As well known in the art, in a semiconductor device fabrication process, a functional layer composed of an insulating film and a functional film is formed on the front side of a substrate such as a silicon substrate, and a plurality of devices such as integrated circuits (ICs) and large scale integrations (LSIs) are formed like a matrix from this functional layer, thus obtaining a semiconductor wafer having the plural devices. The functional layer is partitioned into the plural devices by a plurality of crossing division lines formed on the front side of the semiconductor wafer. The semiconductor wafer is divided along these division lines to obtain individual semiconductor device chips individually corresponding to the plural devices.
In recent years, a semiconductor wafer intended to improve the processing performance of semiconductor chips (devices) such as ICs and LSIs has been put into practical use. This semiconductor wafer is composed of a substrate such as a silicon substrate and a functional layer formed on the front side of the substrate, wherein the functional layer is composed of a low-permittivity insulator film (low-k film) and a functional film formed on the low-k film, the functional film forming a plurality of circuits. Thus, the semiconductor devices are formed from the functional layer. The low-k film is formed from an inorganic film of SiOF, BSG (SiOB), etc. or an organic film such as a polymer film of polyimide, parylene, etc.
Division of such a semiconductor wafer along the division lines is usually performed by using a cutting apparatus called a dicing saw. This cutting apparatus includes a chuck table for holding the semiconductor wafer as a workpiece, cutting means for cutting the semiconductor wafer held on the chuck table, and moving means for relatively moving the chuck table and the cutting means. The cutting means includes a spindle adapted to be rotated at high speeds and a cutting blade mounted on the spindle. The cutting blade is composed of a disk-shaped base and an annular cutting edge mounted on one side surface of the base along the outer circumference thereof. The annular cutting edge is an electroformed diamond blade formed by bonding diamond abrasive grains having a grain size of approximately 3 μm, for example.
However, the low-k film mentioned above is different in material from the substrate of the semiconductor wafer, so that it is difficult to cut the substrate together with the low-k film by using the cutting blade. That is, the low-k film is very brittle like mica. Accordingly, when the semiconductor wafer having the low-k film is cut along the division lines by using the cutting blade, there arises a problem such that the low-k film may be separated and this separation (delamination) may reach the devices (circuits) to cause fatal damage to the devices.
To solve this problem, Japanese Patent Laid-open No. 2005-64230 discloses a wafer dividing method including the steps of applying a laser beam along both sides of each division line on a semiconductor wafer to form two laser processed grooves along each division line, thereby dividing the functional layer (functional layer dividing step), and next positioning a cutting blade between the two laser processed grooves formed along each division line to relatively move the cutting blade and the semiconductor wafer, thereby cutting the semiconductor wafer along each division line. When the laser beam is applied to the front side of the wafer along each division line, debris may scatter from the wafer. To cope with this problem, the front side of the wafer is covered with a protective film formed of a water-soluble resin such as polyvinyl alcohol (PVA), before performing the functional layer dividing step mentioned above, thereby preventing the debris scattered from adhering directly to the devices.